The present invention relates to systems for powering implanted devices. The invention has particular utility for systems for powering current implanted devices requiring 24/7 operation and tens of watts of electrical power for applications such as heart-assist devices and will be described in connection with such utility, although other utilities are contemplated.
The present invention addresses a critical barrier to a major increase in the availability of heart assist-devices to patients in need: the present method of providing 24/7 continuous power to these devices, either as bridges to transplants or as permanent implants. Many publications cite the shortcomings of the existing method, which uses percutaneous links to provide electrical power to Mechanical Circulatory Support Systems (MCSS), for example, left- or right-Ventricular Assist Devices (VADs). These include Slaughter and Myers (2010), Si et al (2008), Danilov et al (2008), and Franco and Verrier (2003). The percutaneously placed wires provide pathways for infection (Franco and Verrier place the rate of infections at 40-45%), they periodically break, potentially create adhesions, and they limit the life style of patients because of measures they must take to avoid infections. A 2001 news release from NIH about the 1998-2001 REMATCH clinical study of percutaneously powered LVADs cited the probability of infection within 3 months of implantation to be 28%. As a result, at this time, the use of VADs is limited to bridge-to-transplant patients, those with extreme loss of heart capability.
Wireless Transcutaneous Energy Transfer (TET) across tissue is the much-preferred, less-invasive method of providing power to these devices. The impacts of a TET power system are that it 1) overcomes a major disadvantage of the present percutaneous method of providing power, namely high susceptibility to infection, opening up a lifesaving technology to hundreds of thousands who suffer from heart failure, and 2) supports the increased use of presently implanted heart-assist devices, and 3) fosters new devices targeted to improving human health.
Powering of MCSS over long periods of time solely by implanted batteries is not possible with the batteries available today because of the continuous high power requirement, which in turn dictates a large storage capacity and heavy battery. A TET system could deliver power directly to the application, while also charging an implanted battery which could take over for periods of 1 to 2 or a few hours. Over the past 50 years much effort has been expended in trying to make an electromagnetic method of TET (EM-TET) work for MCSS. U.S. Pat. No. 6,579,315, to Weiss discloses an EM-TET system for an artificial heart. U.S. Pat. No. 5,630,836 to Prem discloses an EM-TET system for both an artificial heart and a ventricular assist device. Papers (Mehta et al., 2001; Schuder, 2002; Slaughter and Myers, 2010; Danilov, 2010) disclose elements of an EM-TET system and even some clinical trials. Nevertheless only a few if any devices based on this principle are commercially realized. Issues that hold back EM-TET adoption include 1) heating of tissue due to misalignment of transmitter and receiver coils which expose metal to magnetic and electric fields that cause eddy-current heating, 2) heating due to losses in the coils, 3) loss of transmission efficiency with depth of penetration, due to decreased coupling of transmitter and receiver, and 4) decoupling due to perturbation of the inductance of the coils when they interact with nearby metallic or magnetic materials.
U.S. Pat. No. 8,082,041 (Radziemski) describes an ultrasound system suitable for providing low power to devices such as pacemakers, defibrillators and neurostimulators, primarily to recharge implanted batteries. It is well known in the art that batteries for such low power devices are charged for periods of minutes to hours at a rate of once per day to once per month or even less frequently. The patent also contains a description of the prior art with regard to medical-ultrasound power transmission, which is included by reference. These applications typically require a few Watts of input power and typically less than a half-Watt of power at the point of application and do not require addressing the new issues which must be resolved for high-power applications. Specifically the aforesaid patent teaches a bio-implantable energy capture and storage assembly, including an acoustic energy transmitter for contact with the skin, and an acoustic energy receiver converter for converting acoustic energy to electric energy; a battery or capacitor connected to the energy converter; signals upon which one may base alignment of transmitter and receiver; and a method of cooling the assembly. The acoustic energy receiver/converter, which employs ultrasound, is contained within a biocompatible implant.
Although methods for providing signals for alignment of transmitter and receiver are taught in Radziemski, the actual physical methods of aligning those elements is not taught. Absent any electronics to perform the alignment, the only option is that it would be performed manually, by physically adjusting the orientation of the external transmitter unit. In fact that is the present state-of-the-art method for the low power EM-TET method used commercially. In contrast, here is taught a 24/7 high-cooling-capacity element plus a 24/7 non-mechanical alignment system. Such an alignment system is required because solely manual alignment of transmitter and receiver over 24 hours of each day is wholly impractical and unsafe.
The cooling methods taught in Radziemski only included thermoelectric, disposable, or reusable coolers on the transmitter side, and phase change materials in the receiver. These would not be useful for the 24/7 continuous high-power operation needed for MCSS applications. This application requires one to two orders of magnitude more power than the applications discussed in Radziemski, typically at this time 10 Watts, or 20 Watts or more of electrical power at the device to be powered. This in turn, because of the finite efficiencies of all the steps, requires 40, 50, or 60 or more Watts of acoustic power from the transmitter unit. These levels of power require new and novel approaches for safely cooling tissue. Completely passive cooling methods alone, such as a disposable liquid coolant pack as taught in Radziemski, cannot dissipate the substantial 24/7 heat load generated in this high-power application, because such a pack would need to be changed and reapplied an undue number of times a day—making it wholly impractical. Phase change materials in the receiver implant cannot perform continuous cooling in that location because, once the transition has been made, they need time and lower temperatures to regain the previous phase. That time is not available in the 24/7 operation of a MCSS. A thermoelectric cooler as taught in Radziemski is also unusable in the present application because, as is well known in the art, it generates heat itself in proximity to itself and the skin which is dangerous to the patient and adds to the proximate heat load. Likewise cooling systems that operate in the transmitter, such as taught in Sliva (U.S. Pat. No. 5,560,362) do not apply here because those were single-ended systems used in imaging, with low heat loads and low cooling capabilities, and designed to operate pulsed with low duty cycles, not 24/7. They could cool the upper layer of the skin, but would not propagate deeply enough to cool a heat-source in the receiver and the tissue adjacent to it, as must be performed in the present application. Hence there exists a compelling need for an ultrasound delivery system that can deliver 10's of Watts of electrical power continuously while providing a) reliable non-mechanical alignment system, and 2) sufficient cooling capacity to dissipate potential tissue damage.
Although the present state of the art is to require 10 or more Watts to the MCSS, with efficiency improvements in the future, the requirements could be reduced to 5 Watts or less. Also MCSS placed in infants or young children may require less power as well. In those cases the demands on US-TET MCSS power delivery and heat removal will be correspondingly reduced, for example to 5 Watts and 3 Watts respectively.
The invention described here is a modality for transferring energy at a high rate (e.g. power) wirelessly and safely across the skin in quantities sufficient to directly power energy-intensive implantable medical devices.
There are few prior references to using ultrasound as a carrier of energy at the levels needed in heart assist devices. Suzuki, et al (2003) describe a hybrid magnetic-ultrasonic device that employs magnetostrictive materials to generate the pressure waves that carry energy across the skin. That paper mentions ultrasound, but refers to a different and more complex system that only demonstrated ˜5 W of output power. High power ultrasound non-medical applications are well known in that field.
An important theoretical and practical advantage of US-TET is the ability to mitigate the effects of lateral and angular misalignment by non-mechanical electronic means via a two dimensional array of transmitter transducers, leading to a completely self-aligning system that does not require patient intervention. Also, the ultrasound beam, in the near field which is our case, does not diverge significantly, hence losses due to depth of the implant are minimal. Both of these advantages accrue to ultrasound because of its wave nature, and the fact that for power transfer, the ultrasound wavelength at useful frequencies is much smaller than the dimensions of the ultrasound transducers. In EM-TET the converse is true, ruling out the use of non-mechanical alignment by this principle. Willis (US2008/0294208) teaches the use of a two-dimensional ultrasound array to search for a receiver located in or on the heart and provide pacing level voltages to the heart wirelessly. Willis (U.S. Pat. No. 8,364,276) estimates the energy per pacing pulse provided as 0.17 microJoules in a 0.5 millisecond pulse. Assuming a pulse rate of 60 per second, this converts to an average power of 0.17 microWatts. TET-MCSS applications need on the order of 10-20 Watts continuously (10-20 Joules per second). Hence Willis' array without cooling could not be used in the present application. Also, in the MCSS application there is no need for a location function or signal. Willis is trying to find a small receiver some variable distance or orientation with respect to the transmitter array. In the MCSS application the plane transmitter and receiver faces will likely be 10 to 50 mm apart, and very closely parallel to begin with, their diameters being up to 75 mm or more, hence significantly larger than the distance separating them. An unfocused beam will suffice to correct any misalignment by changing the angle of the transmitted wave front so that it is incident upon the receiver closely parallel to the plane face of the receiver, thereby optimizing power delivery.
It is thus an object of the present invention to provide new and novel wireless power transfer techniques which alleviate distress, pain, complications, and operations associated with infections suffered by patients who would instead have to use the present method of power delivery to heart assist devices.